A) Field of the Invention
The present invention relates to an optical device in which light propagating in an optical waveguide is made to have a narrow spectrum by coupling the light with diffraction gratings.
The present invention further relates to an optical device having the structure of raising a coupling coefficient between light propagating in an optical waveguide and diffraction gratings.
B) Description of the Related Art
With a tremendous increase in Internet demands, efforts geared to ultra high speed and large capacity optical communication and optical transmission are intensified. For the Ethernet (registered trademark) having more than gigabit transmission band, a semiconductor laser diode has been sought which is low-cost and capable of direct modulation of 10 Gb/s or higher in an uncooled state. Examples of a semiconductor laser diode capable of meeting this requirement include a distributed-feedback (DFB) laser diode.
In order to manufacture a DFB laser diode at a low cost, a ridge-waveguide distributed-feedback laser diode is promising which can be manufactured by performing crystal growth once, i.e., second crystal growth after an etching process is not necessary. In forming a diffraction grating for distributed-feedback on a ridge-waveguide distributed-feedback laser diode, forming a diffraction grating on both sides of a ridge has a greater advantage in terms of manufacture cost than forming a diffraction grating in crystal.
FIG. 24 is a perspective view of a conventional ridge-waveguide DFB laser diode. An active layer 501 and a clad layer 502 are sequentially laminated on a semiconductor substrate 500. A ridge 503 extending along one direction is formed on the clad layer 502. A diffraction grating 504 is formed on both side surfaces of the ridge 503. The active layer 501 under the ridge 503 functions as an optical waveguide.
FIG. 25 shows another example of the conventional ridge-waveguide DFB laser diode. Although the diffraction grating 504 of the ridge-waveguide DFB laser diode shown in FIG. 24 is formed on both side surfaces of the ridge 503, in the example shown in FIG. 25 a diffraction grating 504A is formed on flat surfaces on both sides of a ridge 503, in place of the diffraction grating 504. The other structures are the same as those of the laser diode shown in FIG. 24.
FIG. 26 shows a position relation between light propagating in an optical waveguide and diffraction gratings. Diffraction gratings 504 or 504A are disposed on both sides of a ridge 503. An intensity distribution of light of a fundamental transverse mode has a maximum at the center of the ridge 503 in a width direction and the intensity gradually lowers as the position recedes from the center, as indicated by a solid line 510. An intensity distribution of light in a first high order transverse. mode (hereinafter, abbreviated as “second order transverse mode”) has a minimum at the center of the ridge 503 in the width direction, gradually increases its intensity and has maximum values on both sides of the ridge, as indicated by a solid line 511. In an area outside the positions at the maximum values, the intensity of light lowers monotonously as the position recedes from the center of the ridge 503.
Since the diffraction grating is not disposed near at the center of the ridge 503 but disposed on both sides of the ridge 503, the intensity of light in the second order transverse mode in an area where the diffraction gratings are disposed is higher than that of light in the fundamental transverse mode. Therefore, a coupling coefficient between the second order transverse mode and the diffraction gratings is about 1.5 to 2 times as that between the fundamental transverse mode and the diffraction gratings. Oscillation in the second order transverse mode is therefore likely to be generated.
In order to lower the coupling coefficient between the second order transverse mode and the diffraction gratings, it is necessary to make the ridge 503 thinner and to bring the diffraction gratings closer to the center of the ridge 503. However, as the ridge 503 is made thinner, electric resistance of the laser diode increases. Making the ridge 503 thinner results in an increase in power consumption and a reduction of optical output to be caused by heat generation during large current injection.
JP-A-2003-152273 discloses a semiconductor laser diode capable of suppressing higher order transverse modes.
FIG. 27 is a horizontal cross sectional view showing a ridge portion of a semiconductor leaser diode disclosed in JP-A-2003-152273. A diffraction grating 521 is formed on both side surfaces of a ridge 520. An optical absorption layer 522 of InGaAs is formed on the outer concave and convex surfaces of the diffraction grating 521, the optical absorption layer being absorbent to oscillation light. The optical absorption layer 522 absorbs light in higher order transverse modes more than in the fundamental transverse mode, so that oscillation in the higher order modes can be suppressed.
Next, description will be made on a transverse mode of light in a laser diode having a buried waveguide (hereinafter called a buried-heterostructure laser diode).
FIG. 28 is a perspective view of a conventional buried-heterostructure laser diode. A mesa portion 551 extending in one direction is formed on the surface of a semiconductor substrate 550. A diffraction grating 552 is formed on the upper surface of the mesa portion 551, and an active layer 553 is formed on the diffraction grating 552. A burying layer 555 is formed on a flat surface on both sides of the mesa portion 551. A current confinement layer 556 is formed on the burying layers 555. An upper clad layer 557 covers the active layer 553 and current confinement layers 556.
FIG. 29 is a graph showing a relation between the width of the active layer and the coupling coefficient between light and the diffraction gratings of the buried-heterostructure laser diode shown in FIG. 28. A curve a indicates the coupling coefficient of the fundamental transverse mode, and a curve b indicates the coupling coefficient of the second order transverse mode.
Since the diffraction grating 552 is disposed on the whole surface of the active layer 553 in the width direction, the coupling coefficient a of the fundamental transverse mode is principally higher than the coupling coefficient b of the second order transverse mode. The second order transverse mode will not oscillate if the width of the active layer 553 is 1.4 μm (cutoff width) or narrower.